Words, in plain language

A

adenosine: A natural substance in the brain that helps to regulate sleep and is thought to add to the need for sleep when we stay awake for longer than usual. Caffeine works by blocking the places in the brain where adenosine acts.
antihistamine: A medicine that blocks histamine, a chemical that helps keep you awake. Some older ones make you drowsy and are sold as nighttime sleep aids.
augmentation: A worsening of restless legs caused by the medicine used to treat it, most often a dopamine drug: symptoms start earlier in the day, come on faster or spread to other parts of the body.

B

benzodiazepine: A family of sedative medicines, such as diazepam and temazepam, prescribed for anxiety and sometimes for sleep. Taken for a long time, they can lead to dependence.
body clock: The timekeeper in your brain that runs on a cycle of about a day, setting when you feel sleepy and when you feel alert. Daylight, especially in the morning, keeps it in step with the clock on the wall.

C

caffeine withdrawal: The headaches, tiredness and low mood that can follow when someone who has caffeine most days stops suddenly. It usually passes within two to nine days, and it isn’t considered dangerous.
cataplexy: A sudden spell of muscle weakness while awake, usually set off by laughter or another strong feeling, during which the person stays fully aware. It's a sign of narcolepsy type 1.
CBT for insomnia: Cognitive behavioral therapy for insomnia: a short course of practical, talking-based techniques that help retrain sleep. It’s the first treatment doctors recommend for long-term insomnia.
cognitive shuffling: A bedtime mental game: picturing a string of unrelated, everyday things one after another, for example things that begin with each letter of a chosen word.
cortisol: A hormone that helps the body respond to stress and to waking. Its level rises and falls each day: lowest around midnight and highest in the early morning.
CPAP: Continuous positive airway pressure: a machine that sends a steady flow of air through a mask to keep your airway open while you sleep. It's the usual treatment for obstructive sleep apnea.

D

deep sleep: The slowest, most settled stage of sleep, when it’s hardest to wake someone. Most of it comes in the first part of the night.
delayed sleep phase: When your body clock runs late, so sleep doesn't come until well after a usual bedtime and waking early is very hard. It's common in teenagers.
dependence: When your body has adapted to a medicine or substance so that stopping it brings unpleasant effects for a while, such as worse sleep.

F

ferritin: A protein that stores iron, measured with a blood test. The level shows roughly how much iron the body has in reserve.

H

half-life: The time it takes the body to clear half of something, such as caffeine. After one half-life, half is left; after two, about a quarter.
hot flash: A sudden wave of heat through the body, often with sweating and a flushed face, common around menopause. At night it can wake you, damp and too warm.
hypnagogia: The drifting, half-dreaming state between waking and sleep, when thoughts loosen and pictures arrive on their own.

I

insomnia: Trouble falling asleep, staying asleep or waking too early, that keeps happening and affects how you feel or get on during the day.

J

jet lag: The tiredness and broken sleep that come from crossing time zones faster than your body clock can adjust to the new local time.

L

light sleep: The lighter stages of sleep, the drifting doze a small sound can lift you out of. A night spends more time here than in any other stage.
lux: A measure of how much light falls where you are. Daylight outdoors is many times brighter than ordinary indoor lighting, even on a cloudy day.

M

melatonin: A hormone your brain releases as darkness falls, telling the rest of your body that night has come. It's also sold as a tablet.
microsleep: A brief spell of sleep that can happen while someone is trying to stay awake, often without their noticing.

N

narcolepsy: A long-term condition in which the brain can't keep sleep and waking properly apart, bringing overwhelming sleepiness by day and, for some, sudden muscle weakness.
night terrors: Spells in which someone, most often a child, seems to wake in terror from deep sleep, crying out or thrashing, without really being awake. They usually remember nothing of it in the morning.
nightmare: A frightening dream that wakes you, usually from REM sleep later in the night, and that you can often remember clearly afterwards.
nocturia: Waking from sleep once or more in the night to pass urine. It’s common in both men and women, and becomes more common with age.
non-REM sleep: All of sleep except REM sleep, from the lightest stages down to deep sleep.

O

orexin: A messenger made by a small group of brain cells that helps keep us awake through the day and holds sleep and waking steady. It's also called hypocretin.

P

placebo: A dummy treatment, such as a pill with nothing active in it. Studies compare a real treatment against one to see how much of the effect comes from the treatment itself.
progressive muscle relaxation: A relaxation exercise in which you tense one group of muscles for a few moments and then let it soften, moving slowly through the body.

R

rebound insomnia: Sleep that is worse than usual for a night or two after stopping a sleeping pill that had been taken regularly.
REM rebound: A return of extra dreaming sleep after it has been held back, for example by alcohol or a short night.
REM sleep: Rapid eye movement sleep, often called dreaming sleep: the stage when most vivid dreaming happens. The eyes move quickly beneath the lids while most of the body’s muscles stay relaxed and still.
restless legs syndrome: An urge to move your legs, often with uncomfortable feelings in them, that comes on when you rest, especially in the evening, and eases when you move.

S

sedative: Something that slows activity in the brain and makes you feel drowsy.
shift work disorder: Poor sleep, and sleepiness that gets in the way at work, brought on by working nights or other hours that cut across the usual time for sleep. Doctors can assess and treat it.
sleep apnea: A condition in which the throat repeatedly narrows or closes during sleep, briefly stopping or reducing breathing. Loud snoring, gasping and daytime sleepiness are common signs.
sleep cycle: One round of the stages of sleep, from light sleep down to deep sleep and on to dreaming sleep. Each lasts about ninety minutes on average, though the length varies, and a night holds four to six of them.
sleep hygiene: Everyday habits often suggested for better sleep, such as keeping caffeine to the first part of the day, sticking to regular hours and sleeping in a dark room away from noise.
sleep inertia: The groggy, slow feeling just after waking, before the brain is fully up to speed.
sleep paralysis: A brief spell, on falling asleep or waking, of being aware but unable to move the body, while the eyes move and breathing goes on as usual.
sleep pressure: The body’s need for sleep, which builds the longer we stay awake and eases while we sleep.
sleep restriction: A part of CBT for insomnia that limits time in bed to about the amount of sleep you're getting at the moment, then widens it as sleep becomes more solid. It's usually guided by a clinician or a tested program.
sleep study: A night with sensors recording your breathing, brain waves, heart and movements as you sleep, in a sleep clinic or sometimes at home, to see what's happening in your sleep.
sleepwalking: Getting up and moving about while still partly in deep sleep. The person usually remembers little or none of it.
stimulus control: A part of insomnia treatment that helps the bed feel linked with sleep again: going to bed when sleepy, and getting up for a while when sleep won’t come.

T

tolerance: When a medicine or substance does less than it used to at the same dose, because your body has adapted to it.

W

withdrawal: The body’s reaction when a drug it has adjusted to, such as a sleeping pill, another sedative or alcohol, is cut down or stopped too quickly. It can bring anxiety, shaking and sleeplessness, and at its most severe seizures, so stopping is planned with medical support.

Z

z-drug: A group of prescription sleeping pills, such as zolpidem, that act on the same calming system in the brain as older sedatives like benzodiazepines.